Don't pick up abandoned USB flash drives that may be found in public places, or corporate grounds... and Don't plug them into your computer or devices.
If you plug one in, you may think you're helping an owner of a lost USB flash drive by finding a clue of the owner. Don't. Your good intentions may introduce a Trojan, malware or worse, spyware into your device or computer.
Your good deed isn't worth the cost of the flash drive to anyone, especially you. That is, if your files, device data, and personal information on your devices are important to you.
Remember to Encrypt your own Flash Drives. Also Label the outside of your Flash Drives so they may be returned to you, without someone having to plug it into a device to find your name (and see your files).